This invention relates generally to an ink-on-demand type ink jet printer head in which ink droplets are jetted through a nozzle for printing. A variety of ink-on-demand type ink jet printer heads have been proposed in the prior art. Typical examples of such ink jet printer heads are the Kyser apparatus, described in U.S. Pat. No. 3,946,398, and the Stemme apparatus, described in U.S. Pat. No. 3,747,120.
The Kyser system is briefly described with reference to FIG. 1 wherein the apparatus includes an electromechanical transducer 1 having piezoelectric elements 2,3. The transducer 1 is disposed in a recess 4 formed in a substrate 10 thus forming one side wall of a pressure chamber 7 for holding ink. A substrate 5 includes an ink supply path 6, the pressure chamber 7 and a nozzle 9. In combination, the substrates 5,10 form a ink jet printer head. When an input signal is applied to the input terminals 8, the transducer 1 is displaced inwardly as indicated by the broken line and the arrow to decrease the internal volume of the pressure chamber 7, thereby causing an ink droplet to be ejected from the nozzle 9. These are the fundamental design and operating principles of an ink-on-demand type ink jet printer head.
The Stemme apparatus is discussed briefly with reference to FIG. 2, wherein the ink jet printer head includes a piezoelectric element 14, a first pressure chamber 17 which is connected through a path 11 to a second pressure chamber 19 and to a nozzle 13, and an ink supply path 12 for feeding ink from an ink tank (not shown) to the second pressure chamber 19. When an input signal is applied to the input terminals 18, the piezoelectric element 14 is driven so as to decrease the volume of the first pressure chamber 17 causing ink in the chamber 17 to pass through the opening 11 and the second chamber 19, and then to be jetted in the form of droplets from the nozzle 13. This is the fundamental design and operating principles of the so-called double cavity system.
Ink droplets can be ejected at a high frequency and a plurality of chambers, nozzles, and driving transducers can be arranged in a single compound head to provide a row or several rows of closely spaced dots in the known manner. However, at high printing rates, for example, in excess of 500 Hz, an ink layer forms on the front face of the ink jet printer head in the vicinity of the nozzle openings. The droplets ejected from the nozzles pass through the surface ink layer and are deflected in their path. For this reason these ink jet printer heads suffer a disadvantage in that print quality is degraded. That is, the printing intervals are not regular because the ink droplets are ejected along a slanted or deviant axis from the axis of the nozzle. Furthermore, this makes it necessary that the ink jet printer head, and more particularly, the nozzles, to be set as close to the printing sheet or other recording medium as possible so as to minimize the dot shift due to the slanted trajectory of the droplets.
What is needed is an ink jet printer head which ejects ink droplets at a high frequency without dot shifting so that print quality is high and the nozzle openings need not be very close to the medium being printed upon.